1. Field of the invention
The present invention relates to a control system for an automatic transmission of a motor vehicle, and more specifically to a line pressure control for a hydraulic control mechanism of the transmission.
2. Description of the prior art
Generally, an automatic transmission of a motor vehicle is provided with a torque converter, e.g. a transmitting mechanism employing a planetary gear mechanism. The transmission is also provided with a plurality of frictional elements, such as clutches, and brakes for establishing a desirable shift gear stage among the plurality of shift gear stages provided in the transmission. For this purpose, a hydraulic control circuit is provided with a plurality of solenoid valves for switching hydraulic passages in the circuit so that the frictional elements are engaged and disengaged to perform a desirable shift operation.
Japanese Patent publication No. 54-2349, issued to Ford Motor corporation and published for opposition on Feb. 6, 1979, discloses a basic electrical hydraulic control system for an automatic transmission in which regulator valves are provided for controlling a line pressure of hydraulic passages of circuit of the transmission, and pilot pressures of the regulator valves are controlled by solenoid valves.
Switching operation of the hydraulic passages of the hydraulic circuit must be done timely done; otherwise, there might be produced a torque shock resulting from unusual change in engine and/or turbine speed.
In view of the above, there has been proposed controlling engaging force of the frictional elements to obviate the torque shock during shift operation of the transmission. For instance, Japanese Patent Public Disclosure No. 56-10851, laid open to the public in 1981, discloses a hydraulic control in which the hydraulic pressure introduced into the frictional element is controlled to gradually change the engaging force of the frictional elements and to reduce the difference between a target time period for shift operation and an actual time period thereof.
Japanese Patent Publication Nos. 49-30051 published for opposition on Aug. 9, 1974, and 61-48021, corresponding to U.S. Pat. No. 4,502,354 and published for opposition on Oct. 22, 1986, disclose similar control system for obviating the torque shock due to the shift operation.
Meanwhile, the frequency of control signal for the solenoid valves is determined, taking account of reliability of operation, durability thereof, and the like. It should be noted that as the frequency of the control signal is decreased, the response in the line pressure control is deteriorated, but that if the frequency is too high, the durability and the reliability of the solenoid is badly affected.